1. Field of the Invention
The present invention relates to a power feeding coil unit and a wireless power transmission device for wireless transmission of power.
2. Description of the Related Art
Wireless power transmission technology that utilizes an electromagnetic induction effect between a primary (power feeding) coil and a secondary (power receiving) coil that face each other to transmit power without any mechanical contact such as a cable has attracted attention recently.
The application of the wireless power transmission technology described above to, for example, secondary battery chargers for electronic devices and the like has been attempted, and there has been an increasing demand for the development of a technology that allows high-efficiency and low-loss power transmission.
To meet such a demand, for example, Japanese Unexamined Patent Application Publication No. 01-23730 proposes a charger including a coil unit for excitation which includes a plate-shaped coil yoke and a pair of charging coils arranged in a planar array on the coil yoke so that magnetic poles are created vertically, and magnetic poles are opposite to each other. Japanese Unexamined Patent Application Publication No. 01-23730 discloses that, in the described charger, the magnetic poles on the individual charging coils are oriented in opposite directions, thereby enabling effective formation of a magnetic circuit and achieving high charging efficiency.
In the application of a wireless power transmission device to a charger for a power electronic device such as an electric vehicle, a large current needs to flow through a charging coil in order to meet the demand for high-power transmission. In the technique disclosed in Japanese Unexamined Patent Application Publication No. 01-23730, if a large current flows through the pair of charging coils on the coil yoke, on which magnetic poles are created vertically, not only the strength of a magnetic field contributing to power transmission but also the strength of an unwanted leakage magnetic field formed in a location away from the charging coils will increase, causing electromagnetic interference which might affect nearby electronic devices and so forth.